Methods of this kind are generally known. Correction frames need to be recorded in particular when using digital solid state detectors for X-ray imaging, since uncorrected X-ray images cannot be used for diagnostic purposes. It is only after X-ray images have been corrected with the aid of such correction frames that said X-ray images can realistically be post-processed using organ-specific image processing software and can then be diagnostically interpreted.
The necessity for correction frames arises from the physical characteristics of the solid state detectors used for X-ray imaging. In the case of said solid state detectors the optical and electrical properties of the individual pixels can show marked variations. For instance the leakage currents of the photodiodes and the switching transistors used for readouts can vary. Furthermore resistors and capacitors used for readouts can vary row by row and column by column. Also the amplifiers used for readouts can be embodied in different designs. Moreover the solid state detectors are occasionally made up from different sub-matrixes that exhibit different properties. These and other circumstances are the reason why among other things what is referred to as the offset varies widely across solid state detectors.
Furthermore the offset of the individual pixels is not constant, but varies over time. These variations can be provoked by temperature fluctuations, such as temperature changes in the sub-matrixes or temperature fluctuations in the electronic modules, or can also be dependent on the operating modes in which the solid state detector is operated. For example the offset can depend on the frame frequency, which in the case of fluoroscopy typically varies between three and sixty images per second. In other applications, images with X-ray windows of up to 2.5 seconds are recorded every half to three seconds. This is the case for instance with blurred image tomography. The offset can however also depend on the radiation dose, the readout mode, the readout range used and the radiation period.
Due in particular to the strong dependency on temperature, offset frames need to be created contemporaneously in relation to the X-ray plates being recorded in different operating modes or sequences. This is particularly true in the case of uncooled solid state detectors, since the variations in their temperature depend on the ambient temperature and the operating mode of the solid state detector concerned.
Furthermore devices for recording high energy images support a considerable number of operating modes. X-ray equipment for vascular angiography or cardiology can typically be operated in 30 to 40 different operating modes. Frequently it is not enough to take only one offset frame for all the various operating modes. Instead, up to 60 offset frames are recorded in order to suppress noise as far as possible. In the operational pauses between acquisitions of X-ray images of an object under investigation there is often not enough time available to create offset frames for all operating modes.